Systematic investigation of pseudogaps in In, Al, and Pb islands

The novel pseudogap state has been systematically studied on flat-top In, Al, and Pb islands/films grown on $\mathrm{Si}(111)$ surfaces with scanning tunneling microscopy. Our observations and analysis suggest that the interplay between electron-phonon interaction and quantum confinement, the dynamic Coulomb blockade, and the orthodox Coulomb blockade are, respectively, the dominant mechanism of the formation of such pseudogap states in the flat-top metal islands at different island size scales. Our results can help to settle the dispute on the origins of the pseudogap state in such systems.